The present invention relates to a method and a device for decoupling of the behavior of an aircraft, particularly a helicopter.
In the context of the present invention, coupling of the behavior is understood to mean the interaction which exists between various controls and/or attitudes of an aircraft, such that a change to a control (for example the actuation of the pitch control column of a helicopter with a view to altering the pitch trim) or of an attitude entails not only an associated corresponding alteration of the behavior of the aircraft (representative of the said change), but also moreover an interfering action generated on other attitudes.
Hence, a method or a device for decoupling of the behavior of an aircraft, in the context of the present invention, represents a method or a device intended, upon a change to a particular attitude or to a control, to eliminate the interfering action arising from the coupling of the aircraft.
It is known that, on a helicopter, the couplings of the behavior thereof are many and are essentially of two different origins, namely control couplings and axis couplings.
In a known way, the control couplings exhibit very clear-cut and instantaneous effects after action on the controls. A characteristic example is the collective-pitch/yaw coupling. The lift of the main rotor keeps the helicopter in the air. The drag of the rotor blades creates, by reaction, a torque which tends to make the helicopter turn in yaw. In the equilibrium state, this torque is compensated for by that generated by the rear rotor. Hence, if the position of the collective pitch is altered, the drag of the main rotor blades is also altered, and an interfering torque is created, generating the onset of yaw.
Moreover, in a known way, the axis couplings are of very varied origins and exhibit a significant effect appearing after the alteration to the flight configuration, for example a change of speed, incidence or sideslip. A characteristic example is the rotor/empennage interaction. It is known that, between about 50 km/h and 150 km/h, an empennage in the low position lies within the downwash from the rotor. This results in an increase in the pitch-up moment (and of the roll if the empennage is asymmetric).
These couplings of the behavior of the aircraft are consequently very troublesome.
Various methods intended to limit these couplings are known, particularly:
the control coupling can be combated by flat-rate control decoupling. In the case of the "collective-pitch/yaw" decoupling, the movement of the collective lever causes a proportional movement of the yaw control, which is added to the yaw control generated by the pilot of the aircraft; and PA1 the axis decoupling can be performed by a flat-rate feedback to the controls of parameters measured in flight, such as the lateral and vertical load factors, for example. PA1 a) a first control demand is determined, representative of the actuation of a control member; PA1 b) a transfer function is determined, representative of the behavior of the aircraft upon actuation of the control member; PA1 c) at least on the basis of a partial inversion of the said transfer function and of that part of the transfer function not taken into account by the partial inversion, a second control demand is determined, capable of generating the decoupling; PA1 d) an overall demand is calculated from the sum of the first and second control demands; and PA1 e) the overall demand thus calculated is applied to the aircraft. PA1 a control member capable of being actuated by a pilot of the aircraft; PA1 means for determining first control demands as a function of the actuation of the control member; and PA1 means for actuating a member controlled as a function of control demands received, PA1 a calculating unit which, by means of a transfer function representative of the behavior of the aircraft upon the actuation of the control member, determines a second control demand capable of generating the decoupling, at least on the basis of a partial inversion of the transfer function and of that part of the transfer function not taken into account by the partial inversion; and PA1 a calculating means calculating, from the sum of the first and second control demands, an overall demand which is conveyed to the means of actuating the controlled member as a control demand.
However, in practice, by reason of the complexity of the phenomena at play, these methods at best make it possible to attenuate the effect of a limited number of couplings. None of the preceding solutions is therefore totally satisfactory for overall decoupling.